Optical modulators are conventionally known where the intensity of light branched by a coupler has a reduced dependence on the wavelength (hereinafter, “wavelength dependence”). For example, an optical modulator is known that branches light output from a Mach-Zehnder waveguide into a first branch and a second branch by an upstream coupler, branches the second branch into a third branch and a fourth branch by a downstream coupler, and takes the first branch as main signal light and the fourth branch as monitored light. The wavelength dependence of the intensity of the fourth branch branched by the downstream coupler of the optical modulator is the inverse of the wavelength dependence of the intensity of the second branch branched by the upstream coupler (see, for example, Japanese Laid-open Patent Publication No. 2009-145781).
In the conventional optical modulators, however, only one of the branches output from the upstream coupler is transmitted to the downstream coupler, resulting in a high loss.